These little extra details will help the players, too. Not only will they better imagine the critter (it'll stand out more in their minds if it's exceptional in some way), but they can more easily discuss tactics if their foe is easy to name, and can do so in-character too if there's an obvious detail.
This table is mostly useful to games where hoards of monsters or hostile creatures are common, like in fantasy settings such as D&D. I haven't included any system-specific rules, but there are suggestions in the second column (feel free to ignore these if you'd like the detail to be purely cosmetic). Some of the details represent different levels of experience (a critter with one less eye might easily be a veteran), others denote minor biological differences, and could be used to trace genetic lineage ("That must be the brother of the one that ambushed us a week ago!").
|1.||One less eye than normal. If the critter usually only has one eye (cyclops, beholder, etc.), the eye is milky or looks cracked, or it has other notable scars or old wounds.||Minor penalty on ranged attacks, minor bonus on morale.|
|2.||Horns. If the creature is usually horned anyway, it has two sets, or an especially long set.||Gives the creature a bonus attack with the horns.|
|3.||Unusually gnarled-looking skin, maybe scaled or rock-like. If the monster has no discernible skin (gelatinous cube, ectoplasmic ghost) it has a film over it, like custard.||Extra constitution or resistance to physical attacks, minor agility penalty.|
|4.||Musical or noisy. The creature continually barks out a war hymn in its own language or whistles an eery melody (if intelligent), or howls or screams with ear-shattering volume for the entire duration of a combat (if low intelligence or mindless).||Depending on your whim, the critter might gain a sonic attack. Makes it impossible for the creature to hide or surprise once it starts making noise.|
|5.||More teeth. If the critter doesn't usually have teeth or a mouth, it does now.||Gives the creature a bonus attack with the teeth. Might intimidate enemies if particularly unnerving (a slime monster with teeth is a bit more scary than one without).|
|6.||Not the same creature as the rest. You can go subtle with this one, or crazy. Perhaps introduce hybrid creatures or unusual modifiers (a lizardman-orc, or a vampiric gnoll). Sci-fi settings may feature mutations and artificial hybrids, fantasy settings may have magical hybrids.||Modify as appropriate.|
|7.||Marked with runes. If the creature has armour, it can be the armour that is marked; if not, the runes could be carved in the skin, or tattoos. Runes could be sigils, or an ancient language.||Pick your favourite rules modifiers for whatever the runes represent. Alternately, could just denote social superiority.|
|8.||Big bones. The critter is notably more heavy-set than usual, or if not a creature supported by a skeleton is of greater mass than expected.||Extra constitution or hit points, minor agility penalty.|
|9.||Elemental affinity. The monster is surrounded by a field of fire (think phlogiston), or can summon bursts of air at will. Perhaps it is electrical or plasmic in nature and gives shocks to anything conductive that touches it with an appearance like St. Elmo's fire, or is frosted over and immune to cold attacks.||Modify as appropriate.|
|10.||Decorated. This could be in war paint, or fancy clothes or jewellery (if intelligent), or in blood, or the creature could just be an unusual colour.||Slight morale boost to the other creatures around it.|